Preparation of Methyl Orange

Background Reading:

Small Scale Syntheses, pp 363 (azo dyes).McMurry, J., Organic Chemistry, 7th Ed., pp 500-3 (UV-Vis spectroscopy) and 941-5 (diazonium ions and their coupling reactions) [6th Ed.: pp 483-6, 917-21]

Summary

The first step is called “diazotization.” Sodium sulfanilate reacts with sodium nitrite in hydrochloric acid (i.e., nitroso cation) to form an unstable “diazonium salt.”The second step is the “diazonium coupling reaction.” The diazonium ion is used in situ, and reacts with N,N-dimethylaniline to form the acidic azo dye.The crude dye will then be isolated, and used to create dyed nylon fabric.

Reaction Scheme

SO3H

NH2

SO3

NH3

SO3- +Na

NH2

SO3- +Na

NH2

SO3- +Na

N N+

Cl

SO3- +Na

N N+

Cl

N N N(CH3)2Na+ -O3S

N(CH3)2

+

-

+ Na2CO32 2 + CO2 + H2O

Sulfanilic Acid (zwitterion) Sodium Sulfanilate

HCl / NO2-

Sodium SulfanilateDiazonium Chloride

+

N,N-Dimethylaniline Methyl Orange

CH3COOH

N+

Na+ -O3S N H NCH3

CH3

N N N(CH3)2+Na+ -O3SH

OH2

N N N(CH3)2Na+ -O3S

..+

.. ..

.. .. ..

Methyl Orange (azo dye)

Mechanism

Diazotization:

O N OH O N OH2+ O N+

NHO3S

H

H

N ON

+HO3S

H

H

N OOH2

NHO3S

H

N O

OH2

NHO3S N O NHO3S N OH

NHO3S N OH2+ N+

HO3S N

-H2OH3O+

nitrous acid nitroso ion

:

+

N-Nitrosoamine

H3O+

Diazohydroxide

H3O+

.. ..

Diazonium (electrophile)

Diazo Coupling:

Color Theory

The first real attempt to interpret the color of dyes in terms of chemical structure was made by Otto Witt in 1876. He proposed that dyes consist of conjugated -bonded systems, called chromophores, and salt-forming groups, called auxochromes, which are polar substituents that modify their colors.

A chromophore is a simple, unsaturated, e- withdrawing group attached to an aromatic ring system. The extended conjugation due to the chromophore decreases E (and increases max) of the * transition, so that the max is in the visible range. Examples include nitro (-NO2), azo (-N=N-), carbonyl (C=O), and nitrile (-CN).

An auxochrome is an e- donating group (by resonance) attached to an aromatic ring system. These also change E and max. Examples include hydroxyl (-OH) and amino groups (-NH2, -NHR, and –NR2).

The function of a dye is primarily to create a particular color or hue in association with the fabric or substrate. Both natural fibers (like wool, silk, and cotton) and synthetic substrates (like nylon) can be dyed. Dyeing is generally carried out in aqueous solution, and the process of attaching a dye molecule to the fiber is called adsorption.

Dyes that are attracted to the textile by physical forces (physical adsorption, or physadsorption), and have a direct attachment to the fiber (chemical adsorption, or chemisorption), are called direct dyes.

Acid dyes, like methyl orange and tartrazine, are water-soluble anions (anionic dyes) that form ionic bonds between the dye and the fiber. Both wool and nylon are amide macro-molecules that have terminal amino (-NH2) and acid (-COOH) groups. These can form ionic bonds with the dye molecules. As with amino acids, these groups often exist as zwitterions, that is the molecule has both a positively-charged group (-NH3

+) and negatively-charged group (-COO-). Anionic (or acid) dyes have –SO3

- groups that form ionic bonds, or salt linkages, with the – NH3+ groups on

the fibers.

Acid-Base Indicator Properties of Methyl Orange

N N N(CH3)2Na+ -O3S

H+

OH2

N N N(CH3)2Na+ -O3S

H

N N(CH3)2Na+ -O3S N

H

.. .. ..

....

+

Resonance

+..

Protonation (H3O+)Deprotonation (OH-)

Methyl Orange (Azo Dye) pH> 4.4 (yellow w/ max = 462 nm)

Helianthin (Protonated Dye) pH< 3.2 (red w/ max = 506 nm)

Procedure

Pretreatment before Diazotization1. Dissolve 1.2 g anhydrous Na2CO3 with 50 ml DI H2O

in a 125-ml Erlenmeyer flask using a stir bar.2. Add 3.6 g anhydrous sulfanilic acid, and heat solution with

a hot water bath until dissolved. (It may still appear cloudy.) 3. Gravity filter if solids are present, then rinse paper with 3-5 ml of hot water.

Discard filtered solids, and cool filtrate to room temperature.

Diazotization Reaction4. Add 1.5 g NaNO2 to filtrate, and stir until dissolved.5. Add 5 ml conc. HCl to a 400-ml beaker containing

25 ml of (50% / 50%) ice water while stirring. Caution: Conc. acids are corrosive!

6. Pour sulfanilate solution into 400-ml beaker with HCl solution, and place beaker in an ice bath. The diazonium salt will form as a white solid.

Coupling Reaction7. Obtain 2.7 g of N,N-dimethylaniline (d = 0.96 g/ml) in a 10-ml graduated cylinder, and

pour directly into the 400-ml beaker. Wash cylinder with 2.0-ml conc. acetic acid, and add contents to 400-ml beaker. Stir for 15-20 min at 20-25oC to ensure complete reaction.Caution: N,N-dimethylaniline is toxic and is readily absorbed through the skin!

Handle only with gloves! It is also volatile, and needs to remain completely in the hood until transfer is complete!

8. Slowly add 30 ml of 10% NaOH. Check pH of aqueous phase. Add additional NaOH solution, if necessary, until basic.

Isolation of Crude Dye9. Heat to boiling with hot plate for 10-15 minutes. When most of dye is dissolved,

add 10 g NaCl (salting out), then cool crude reaction mixture in an ice bath.10. Vacuum filter, and wash dye twice with 10 ml cold, saturated NaCl solution.

Discard filtrate in sink (~110 ml aq salt waste). 11. Remove 0.5 g of dye, and dry remainder in oven overnight to obtain yield.

Dying Nylon Fabric12. Place 0.5 g of dye in a beaker with 300 ml of tap water.

Add 10 ml of 15% Na2SO4 and 5 drops of conc. H2SO4, then heat to boiling.Caution: Conc. acids are corrosive!

13. Add nylon fabric to bath and stir for 5-10 minutes. Then, remove dyed fabric and rinse well with water.

Data To Collect

Rather than only pasting reactions and mechanisms into the report, describe the mechanisms qualitatively in a paragraph or two.

Determine theoretical yield % by finding moles of the product and of the two reagents. Also, attach a portion of the dyed fabric to your report.

Post-Lab Questions

1. Use the name of the azo compound on pg 945 of McMurry [pg 921, 6th Ed.] to name our product more systematically. (Hints: Precede name with “sodium”, and end name with “sulfonate”. Also, precede “dimethyl” with “N,N-” inside the parentheses.)

2. Suppose we wanted to create the azo compound shown on pg 945. What are the common and systematic names of the aromatic compound that reacts with HNO2 to create the benzenediazonium bisulfate?

3. Describe electronically which part of the methyl orange molecule can behave like a Brønsted base. (See acid-base indicator diagram.)

4. Explain electronically why dyes are colored and how the added proton on methyl orange changes the color (max).(See color theory discussion, as well as pp 502-4 in, McMurry [pp 485-6, 6th Ed.].)

5. How does methyl orange attach to the nylon fabric?